Heretofore, a power transmission apparatus has been commercially available, which utilizes a worm and a worm wheel as the rotating bodies, while the worm wheel is formed in a disc shape, on the circumference of which at its toothed positions are arranged rotatably a plurality of balls for reducing friction loss during engagement.
In such power transmission apparatus, the balls in the engaged phase never slip off but in the disengaged phase readily slip off. In order to avoid such slipping off and to retain the balls rotatably, therefore, a variety of ball-retaining structures has been proposed, for example a structure having arranged balls rotatably embedded more than halfway thereinto, or a structure having an outer annular ring covering the disc circumference with the balls except for the engaged areas in order to prevent the slipping-off of the balls. However, the former structure has the disadvantage of complicated and time-consuming production, while the latter has the problem of very difficult arrangement and adjustment of the outer annular ring at the minimum clearance or gap for preventing the balls from slipping off therethrough. Namely, the outer annular ring must be partially cut out to cover only the non-engaging area, but the partially cutout ring must be arranged as close as possible to the engaged area, which arrangement requires a difficult adjustment.
Accordingly, an object of the invention is to provide a ball-retaining structure of very simple construction but capable of preventing balls from slipping off.